US 20030156057 A1 Abstract In a method for HPRF-radar measurement of the range and Doppler frequency of at least one target, a transmit signal is generated which consists of two pulse sequences that are interleaved on a pulse to pulse basis, and have the same pulse repetition frequency PRF and the same transmit frequency. The pulses of a first one of the two pulse sequences have a linearly increasing phase value with a fixed phase difference φ
_{1n }(greater than zero) from pulse to pulse with φ_{1n}≧0, while the pulses of the second pulse sequence have a linear increasing phase value with a fixed phase difference φ_{2n }which differs from φ_{1n}. The two received base band signals of each individual pulse sequence are Fourier transformed, and the amplitude peaks of the resulting two Fourier spectra are determined. The amplitude peaks of the two Fourier spectra are then clustered to determine pairs of spectral lines of a Doppler frequency shift of
The target Doppler frequency is calculated based on two Doppler frequencies of a previously determined spectral line pair, and the target range is calculated based on the phase difference between the two spectral lines of a previously determined spectral line pair.
Claims(2) 1. A method for HPRF-radar measurement of range and Doppler frequency for at least one target, said method comprising:
generating a transmit signal which includes first and second pulse sequences that are interleaved on a pulse to pulse basis, and have the same pulse repetition frequency PRF and the same transmit frequency, wherein
pulses of the first pulse sequence have a linearly increasing phase value, with a fixed phase difference φ
_{1n }from pulse to pulse, with φ_{1n}≧0; pulses of the second pulse sequence have a linear increasing phase value with a fixed phase difference φ
_{2n }from pulse to pulse, with φ_{1n}≠φ_{2n }and φ_{1n}≧0; Fourier transforming received base band signals of each individual pulse sequence; determining amplitude peaks of resulting two Fourier spectra; clustering the amplitude peaks of the two Fourier spectra to determine pairs of spectral lines of a Doppler frequency shift of calculating target Doppler frequency based on the two Doppler frequencies of a previously determined spectral line pair; and calculating target range based on a phase difference between spectral lines of a previously determined spectral line pair. 2. An apparatus for HPRF-radar measurement of range and Doppler frequency for at least one target, comprising:
means for generating a transmit signal which includes first and second pulse sequences that are interleaved on a pulse to pulse basis, and have the same pulse repetition frequency PRF and the same transmit frequency, wherein
pulses of the first pulse sequence have a linearly increasing phase value, with a fixed phase difference φ
_{1n }from pulse to pulse with φ_{1n}≧0; pulses of the second pulse sequence have a linear increasing phase value with a fixed phase difference φ
_{2n }from pulse to pulse, with φ_{1n}≠φ_{2n }and φ_{1n}≧0; means for Fourier transforming received base band signals of each individual pulse sequence; means for determining amplitude peaks of resulting two Fourier spectra; means for clustering the amplitude peaks of the two Fourier spectra to determine pairs of spectral lines of a Doppler frequency shift of means for calculating target Doppler frequency based on the two Doppler frequencies of a previously determined spectral line pair; and means for calculating target range based on a phase difference between spectral lines of a previously determined spectral line pair. Description [0001] This application claims the priority of 011 27 990.8, filed Nov. 24, 2001, the disclosure of which is expressly incorporated by reference herein. [0002] The invention relates to a method for High Pulse Repitition Frequency (HPRF) radar measurement of target range and Doppler frequency, such as may be used, for example, in an airborne radar system. [0003] The use of a well known radar waveform with two slightly different transmit frequencies is often referred to in the literature as a frequency shift keying (FSK) technique. Using such a technique, target range and velocity can be measured unambiguously even in multi target situations. FIG. 1 shows the two carrier frequencies f [0004] In each CPI of length T, a received pulsed signal is directly down converted with the individual instantaneous transmit frequency, and shows as a-single information the Doppler frequencies of the observed targets. The two base band signals are Fourier transformed, and target line left out will be observed in two adjacent CPI's in the same Doppler bin, but with different phase information. For a simultaneous range measurement, these two phases φ [0005] The range measurement is unambiguous if Δφ<2π. In order to measure target ranges up to R [0006] The basic idea is applicable to the extent that the assumptions of a purely stationary observation are fulfilled. But each target maneuver inside the measurement time 2T will have a large influence on the phase measurement in the adjacent CPI's, and will cause large range errors in the range estimation procedure in accordance with Eq. 1. [0007] One object of the present invention is to provide a method for simultaneously and unambiguously measuring target range and target Doppler frequency for highly mobile targets, even in multiple target situations. [0008] This and other objects and advantages are achieved by the present invention, in which a transmit oscillator generates a single unmodulated frequency. The HPRF pulse sequence is split into two interleaved parts, which are distinguished and modulated with a digital phase shift keying (PSK) modulation scheme as follows: [0009] The pulses of the first pulse sequence have the same phase state or alternatively have a linearly increasing phase value with a fixed phase difference φ [0010] The pulses of the second pulse sequence have a linearly increasing phase value with a fixed phase difference φ [0011] The latter step is important because it renders the down conversion process simple. The PSK technique guarantees unambiguous range measurement and the interleaved pulse sequences make the measurement robust against target maneuver. Before this background all important requirements can be fulfilled with the PSK waveform according to the invention. [0012] Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. [0013]FIG. 1 is a graphic representation of the transmit frequency for FSK-radar as a function of time; [0014]FIGS. 2. [0015]FIG. 3 shows the transmit waveform according to the invention, with the corresponding pulses to the phase steps of FIG. 2; [0016]FIG. 4 is a schematic diagram of a radar transmit/receive system for carrying out the method according to the invention; [0017]FIG. 5 is a constellation diagram of the transmit and receive signal (base band domain) according to the invention; [0018]FIG. 6 shows frequency spectra of the received base band signals containing three targets; and [0019]FIG. 7 illustrates a sampling scheme for signal processing of the received base band signal, wherein m [0020] The digital PSK waveform can be described easily by the constellation diagram of the phase states of the different pulses in the pulse sequence. In a preferred embodiment a distinction is made between a fixed phase modulation of the first sequence, in which all pulses have the same phase state (for example zero phase) for each pulse, as shown in FIG. 2. [0021] For this digital PSK waveform the transmit signal consists of two interleaved pulse sequences with a fixed transmit frequency, but one having a constant phase (first pulse sequence), and the other being modulated by a linear phase modulation technique (second pulse sequence), as shown in FIG. 3. [0022] The phase modulation from pulse to pulse can be realized for example by using the quadrature modulation technique which is well known in digital radio transmission. The amplitude envelope of the waveform is constant, so that no distortion due to nonlinearity of the individual amplifier need be considered. The linear phase modulation of the second pulse sequence is carried so that the phase shift from pulse to pulse satisfies the condition that in the complete pulse sequence the phase rotates exactly (for example) 20 times in the constellation diagram. This condition leads to a shift in the Doppler frequency domain of exactly 20 FFT bins. [0023]FIG. 4 shows the transmitter/receiver block diagram of the radar system on the basis of a quadrature (de)modulator. [0024] The transmit signal s with φ( φ( [0025] The received signal is down converted with the fixed instantaneous carrier frequency f [0026] wherein for φ(t) the above mentioned Eq's 2 and 3 still apply and b refers to the considerably lower amplitude of the receiving signal. [0027] The advantage of the waveform according to the invention is that the phase constellation diagram of the receive signal is shifted compared with the transmit signal only due to the time delay τ or the target range R respectively, as shown in FIG. 5. [0028] The demodulated signal is A/D converted as shown in FIG. 4, and the received base band signal is split into the two signal sequences m [0029] An example representing a 3 target situation is shown in FIG. 6 which contains the frequency spectra of the received base band signals of first (dark gray) and second (light gray) pulse sequences. As can be easily seen from that drawing, for each target there exists a pair of spectral lines (one from the spectrum of the first pulse sequence and one from the spectrum of the second pulse sequence) with a certain frequency shift Δf. [0030] In order to measure target range and speed simultaneously, the following signal processing steps are performed: [0031] calculate Fourier transformation of both receive signals; [0032] search the maximum amplitude peaks in both spectra; [0033] cluster the maxima of both spectra which have a frequency shift of Δf (Doppler bin pair); [0034] calculate Doppler frequency (or target speed) for each Doppler bin pair; and [0035] calculate phase difference for each Doppler bin pair and calculate target range. [0036] These steps will now be described in detail based on a preferred embodiment of the invention. [0037] Transmit signal s [0038] The receive sampled sequence inside a single range gate is considered. The sample values have a time distance of PRF [0039] The amplitude b of the target echo signal is calculated in accordance with the radar cross section (RCS) and in accordance to the radar equation.
[0040] Inside each individual range gate the two received and demultiplexed sequences m [0041] For each range gate spectrum M [0042] For all detected target amplitude peaks of M [0043] A binary factor m [0044] Frequency peaks in both spectra M [0045] In the case of a frequency pair f [0046] The target Doppler frequency can be calculated by the measured frequencies based on the knowledge of which receive pulse sequences contains the zero phase stages (m [0047] The radial velocity of the detected target is in this case:
[0048] The target range R can now be calculated simply by the phase difference directly and the given range gate information: Δφ [0049] The target range ambiguity factor tr [0050] Using the target range ambiguity factor and the range gate information, the unambiguous target range is given by:
[0051] with rg being the number of range gate, where the target has been detected [0052] RGW being the width of range gate [0053] R [0054] System Parameters [0055] The table below shows typical system parameters for an airborne radar system in which the method according to the invention can be used.
[0056] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. Referenced by
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